See page 213 of Dresden Files: Your Story or Combining Skills in the Spirit of the Century SRD
A skill modifies another when a character does something (with the primary skill) in a situation where another (secondary) skill could (or must) help or hinder. This often occurs when doing two things at once.
The character rolls the primary skill with a bonus of +1 if the secondary skill is higher than the primary skill or a penalty of -1 if it is lower. If they are equal there is no modifier.
The bonus or penalty is waived if the secondary skill can only help or hinder.
For example: If Alice needs to throw a knife (Weapons) while balancing on a spinning log (Athletics) then she must roll Weapons, but throwing the knife accurately is harder because she is balancing. If her Athletics is equal to or higher than her Weapons then she can cope without a problem, but if it isn't as good then she gets a -1 penalty to her Weapons.
Another example, using Might, would be a wrestling match. This is hand to hand combat and uses the Fists skill, but strength is more important than usual so Might would modify each participant's Fists.
The terms seem to have been dropped in Dresden, but in SotC they were described as restricting or complimenting the skill if either a bonus or penalty were impossible.
One of the features of Inhuman Might is that it means a character is considered to always have a Might score that is higher than every other skill for the purposes of calculating this modifier. This isn't a generic feature of supernatural powers that you can apply to Inhuman Speed, it is a specific rule for that specific power. (That said, Inhuman Speed has a specific power that lets you ignore the penalty for taking a supplemental action when moving one zone)